1. Field of the Invention
The present invention relates to earth-boring bits of the rolling cutter variety. Specifically, the present invention relates to the cutting structure of earth-boring bits of the rolling cutter variety.
2. Background Information
The success of rotary drilling enabled the discovery of deep oil and gas reserves. The rotary rock bit was an important invention that made that success possible. Only soft formations could be commercially penetrated but with the earlier drag bit, but the original rolling-cone rock bit invented by Howard R. Hughes, U.S. Pat. No. 939,759, drilled the hard caprock at the Spindletop field, near Beaumont, Tex., with relative ease.
That venerable invention, within the first decade of this century, could drill a scant fraction of the depth and speed of the modern rotary rock bit. If the original Hughes bit drilled for hours, the modern bit drills for days. Bits today often drill for miles. Many individual improvements have contributed to the impressive overall improvement in the performance of rock bits.
Rolling-cone earth-boring bits generally employ cutting elements on the cutters to induce high contact stresses in the formation being drilled as the cutters roll over the bottom of the borehole during drilling operation. These stresses cause the rock to fail, resulting in disintegration and penetration of the formation material being drilled. Conventionally, the cutters roll on axes that are offset, or do not coincide with the geometric or rotational axis of the bit. Offset cutters do not purely roll over the bottom of the borehole, but also slide, imparting a gouging and scraping action to the cutting elements, in addition to the crushing mode of disintegration of formation material.
Shear cutting is a disintegration mode that is not taken maximum advantage of in the rolling-cutter earth-boring bit field as it is in the fixed-cutter or drag bit field. Shearing formation material is the dominant disintegration mode in fixed-cutter or drag bits, which commonly employ super-hard, highly wear-resistant cutting elements to shear formation material at the bottom and sidewall of the borehole.
Commonly assigned U.S. Pat. No. 5,287,936, Feb. 22, 1994 to Grimes et al. discloses a shear-cutting gage cutting structure for earth-boring bits of the rolling cutter variety. U.S. Pat. No. 5,282,512 discloses cutting elements for a rolling cutter bit with diamond-charged elements on the forward and central zones of the cutting elements to enhance the shearing or scraping mode of formation disintegration. As shown by U.S. Pat. No. 5,287,936, the shearing mode of disintegration is particularly advantageous employed at the corner and the sidewall of the borehole, where the gage or diameter of the borehole is defined. Maintenance of a full gage or diameter borehole is important to avoid sticking of the bit or other downhole equipment and to avoid the necessity of reaming operations to restore the borehole to the full gage or diameter condition.
A need exists, therefore, for earth-boring bits of the rolling-cutter variety having cutting structures that take advantage of the shearing mode of formation disintegration in addition to the crushing and gouging modes. It is a general object of the present invention to provide an earth-boring bit having a cutting structure adapted to shearingly engage formation material during drilling operation.